1. Field of the Invention
The present invention relates to a gas booster which is connected in series with a large-in-diameter piston to be reciprocatingly driven by pressurized fluid such as compressed air and a small-in-diameter plunger inserted into a plunger chamber, a low-pressure gas introduced into the plunger chamber being boosted according to the sectional area ratio of the piston to the plunger.
2. Description of the Prior Arts
A hydraulic clamp is commonly adopted as the fluid-pressure clamp for fixing a die to a fixed base such as an injection molding machine. The hydraulic clamp is indeed superior in capability of yielding a strong clamping force with a high-pressure oil of approx. 250 kgf/cm.sup.2, which allows the clamping device to be compacted. However, it also allows the oil to leak, although very small in amount, from the high-pressure oil sealing packing, causing the atmosphere to be contaminated therewith. For this reason, the hydraulic clamp has a limit in accomplishing the demand of ultra-cleaning, which is growing pronouncedly in recent years.
The present inventor has previously devised a clamping system as described below with a view to satisfying both the demand of cleaning mentioned above and the compacting of the fluid-pressure clamp. The clamping system is a technique that a compressed air of approx. 5 kgf/cm.sup.2 supplied from an air compressor is boosted to approx. 40 kgf/cm.sup.2 by a gas booster and the resulting high-pressure air is used as a working fluid for the fluid-pressure clamp.
The above-mentioned gas booster is so arranged that a gas pump is driven by a pneumatic piston engine, which is disclosed in U.S. Pat. Nos. 4,042,311 or 4,812,109 previously proposed by the present inventor.
More specifically, in this gas booster, an engine chamber is provided above the pneumatic piston, to or from which engine chamber compressed air is supplied and discharged by a supply/discharge switching means, while a plunger smaller in diameter than the pneumatic piston is provided so as to downwardly protrude from the pneumatic piston, and a plunger chamber is provided below the plunger, with a first check valve for gas intake and a second check valve for gas discharge both connected to a lower portion of the plunger chamber.
The gas booster operates in such a manner that the downward fluid pressure acting on the pneumatic piston from the engine chamber surpasses the upward discharge reaction force acting on the plunger from the plunger chamber to thereby downwardly drive the pneumatic piston; therefore, at the time of high load, when a specified time period has elapsed since the startup of the gas pump so that the discharge pressure has increased to a sufficient extent, the piston and the plunger are driven to descend at low speed. The resulting descending inertia force of the piston at the time of high load is so small that when compressed air is discharged from the engine chamber by the supply/discharge switching means at a time point when the piston has descended almost to the bottom dead point, the piston will be immediately turned over to an ascending return stroke.
In contrast to this, at the time of low load when the discharge pressure is low, the piston and the plunger are driven to descend at high speed; therefore, the descending inertia force of the piston is so great that even though the piston is turned over to the ascending return stroke almost at the bottom dead point, the piston will downwardly overrun the bottom dead point for high load, making the descending stroke of the plunger excessively extend.
In connection with the technical background as described above, a prior art known by the present inventor is such that the depth dimension of the plunger chamber is set so as to be slightly deeper than the descending stroke of the plunger at the time of low load to thereby prevent the plunger from colliding against the bottom wall of the plunger chamber at a final period of the descending stroke of the first piston.
The prior art mentioned above has the following problem.
That is, when the gas pump is at high load, the bottom dead point of the piston is elevated higher than it is at the time of low load, causing the clearance between the bottom wall of the plunger chamber and the bottom end of the plunger to be increased. This in turn causes the compressibility of the plunger chamber to be reduced, with the result that the amount of discharged gas is reduced to an extent of the decrement in the compressibility. The gas booster, as a consequence, is long in the pressure boosting time.